Initialize asm_irq_flags in omap_init_irq and use it in
get_irqnr_and_base to detect between omap7xx and omap15xx/16xx.

Note that both INT_1510_IH2_IRQ and INT_1510_IH2_IRQ are defined
as 0, so use INT_1510_IH2_IRQ for both of them.
Tested-by: NJanusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Signed-off-by: NTony Lindgren <tony@atomide.com>